Its best of you leave candling for the 4th or 5th day, and later.
The first week is a critical time for the delicate germ as it develops. It's best to leave it alone.
"To sum up incubation: place only good, uniform eggs from mature stock in the machines, little end lowest. The temperature should be regulated to about 102 when eggs are placed in the machine, and run
steadily between 102 and 103 degrees during the whole period.*
Turn the eggs twice daily after third day and air slightly after first week, increasing time as germ grows older, but never let them cool below 80 degrees. On the eighteenth day put moisture in the moisture
pan. This should be steaming water so that the humidity is brought up in the machine.
A steady temperature is the prime factor in bringing out a good hatch. Eggs from sturdy stock of mature age, with males and females unrelated, will give best results. As the success of the egg- farmer depends upon the vitality and laying qualities of the pullets, it is imperative that these conditions be adhered to." - - Charles Weeks.
* This was the common temperature given in 1922, when Mr. Weeks wrote this. It was the air temp and he used a still air, kerosene fired incubator. No fans, no water weasels, no modern gadgets.